Treatment of manganese steel



Patented Feb. 6, 1945 TREATMENT OF MANGANESE STEEL .Carl W. Weesner,Warren, Ohio, and Wallace' Bruce Leffingwell, Sharon, Pa., assignors toSharon Steel Corporation, Sharon, Pa., a corporation of Pennsylvania NoDrawing.- Application May 18, 1942,

Serial No. 443,452

8 Claims. 7

The invention relates to a treatment of manganese steel, and moreparticularly to the treatment of high manganese, high carbon steel toeliminate surface brittleness, toincrease its ductility, and to improveits appearanceand surface characteristics.

Difiiculties are encountered in the manufacture of cold rolled sheetsand strips of the thinner gauges from high manganese or "Hadfield steel,that is, steel containing from 10% to 20% manganese and from 1.00% to1.50% carbon; andit is of great importance that these difficulties beavoided or eliminated, because large quantities of such high manganesestrips and sheets are de-.

sired for the manufacture of helmets for the armed forces.

Helmet steel must be soft, ductile and have deep drawing qualities, inorder that helmets may be properly formed or drawn to the desired shapefrom high manganese sheets or strips. The sheets or strips may be asthin as .044 to .033 inch; and cold rolling operations must be performedto roll steel sheets and strips of such gauges. High manganese steelwork hardens very quickly and can ordinarily only be cold rolled to amaximum of about 25% reduction. A heat treatment is then required aftercold rolling to soften the same, and to enable further cold rolling. H

The heat treatment of high manganese, high carbon steel' to soften thesame, or to restore ductility, or to permit further cold rolling afterone cold rolling operation, consists in heating the steel toa-temperature-above the upper critical point, approximately 1800 F. to1900" F and preferably to about 1850 F. in order to dissolve brittlecarbides and produce austenite; and in then quickly cooling the same,preferably by quenching in water.

This is because ductility in high manganese, high carbon steel dependsupon the formation of gamma iron or austenite; and also upon theretention of gamma iron or austenite at room temperatures, obtained by aquick water quench. Also, the rapid cooling, as by the water quench,retains the carbon in solid solution in the austenite at roomtemperatures.

Although heat treatment is necessary to soften the steel and torestoreductility following a cold rolling operation, unfortunatelysteels of the Hadfield types are easily oxidized and decarburized at thesurfaces when heated to the required temperature. Many bad surfaceconditions result from the oxidation or scaling; and the decarburizationof the surface'skin of the both, in varying proportions.

metal sheets or strips, apparently causes surface embrittlement.

The decarburization or lowering of the carbon content of the surfaceskin of the-metal is substantial and the carbon content at thisregion,may become as low as .50% carbon, or less.

When Hadfield steel is thus decarburized it is unable to completelyretain austenite at room temperatures, and on cooling all or part of theaustenite apparently decomposes and forms epsilon iron or epsilon ironand alpha iron, or These decarbwrized surface layers containing epsiloniron and alpha iron are brittle and are formed with a myriad of smallcracks; and the material though generally soft, has a ductility lowerthan desired.

When the metal is subjected to deep drawing operations, the small cracksdevelop and increase in size in the brittle surface layer, giving acharacteristic' granulated or frosty appearance. Thus the heat treatmentwhich is intended to soften the steel and-render it more ductile, on

-, the contrary causes surface embrittlement and s cordance with strippractice.

lowered ductility.

These undesirable surface characteristics are present in high manganesesheet steel made in and stripsheets after hot rolling ordinarily requirea plurality of cold rolling operations, each followed by a heat treatingoperation, to reduce the strip or stripsheet thickness down to say .044to .033 inch.

The formation of the brittle surface layers which are believed to be dueto the presence, particularly, of epsilon iron therein,- iscumulativewith repeated heat treatments following cold rolling,operations, so that the undesirable characteristics increase inmagnitude, the thinner is the ultimate product, in making high manganesestrip or stripsheet steel in accordance .with strip practice. I 1

These difficulties, as stated, result because of the excessive oxidationand decarburization .of

- the surface of the material during the heat treating operations.

These results are contrary to the normal proper- The ultimate result isthat the material has low ductility and an unsatisfactory surface andpoor appearance because of the for.

ties of austenitic manganese steel, which are those of austenite; namelylow elastic limit, but great hardness and toughness and wearing powercombined with much ductility.

Accordingly, the production, of high manganese, high carbon steel inaccordance with strip I practice has been quite limited.

An object of the present invention is therefore to eliminate or removethe brittle surface layers from cold rolled, heat treated, highmanganese steel so'as to remove the epsilon or epsilon and alpha ironcontained therein, and leave manganese steel which is substantiallyentirely austenitic in character, so that the normal properties ofaustenitic manganese steel may be obtained.

A further object of the present invention is to 'provide a commerciallyusable and satisfactory method of cold rolling high manganese steel inaccordance with strip practice to gauges as thin as .044 to .033 inch inthickness.

It would be expected that the undesirable decarburized surface layerscould be removed by. continuing the pickling operation used for removingthe scale after heat treatment, until asumcient amount of the surfacemetal is eaten away or dissolved. However, the ordinary pickling acids,such as sulphuric or hydrochloric acids or. combinationsof the same,cannot be used on high manganese steel for metal removal without leavingan etched or pitted or sharp crag-like surface, in the same way thatthese acids an also nitric acid attack almost all other steels if usedfor metal removal.

However, we have discovered that cold rolled heat treated high manganesesteel strips, sheets and the like may be treated in a dilute solution ofnitric acid, approximately up to 5% nitric acid by weight in water, at atemperature of 140 to 200 F. for from one to approximately six min-'utes depending upon the amount of surface metal to be removed and uponthe concentration and temperature conditions, for removing the scaleformed during the heat treating operation, and

for removing the undesirable decarburized metal surface layers which areextremely detrimental to strength and duciility.

We have further discovered that certain other unexpected results occurwhen high manganese high carbon steel strips and sheets are so treatedto remove scale and the brittle decarburized surface layers. Thus, wehave found that seams, scratches, scale patterns, and rough orundesirable surface defects or imperfections which may be present on thesurface of the. metal due to previous rolling or due to the oxidationand scaling of the metal during heat treatment, are partially or totallyremoved, and the surface is greatly improved with a bright and lustrousappearance. Y

Thus, the resulting surface of the metal'is bright in color, is .freefrom sharp projections or indentations which might tend to injureductility,

and it has a greater usefulness, aesthetic appearance and corrosionresistance by virtue of the removal of undesirable surface conditionsdetrimental to strength and ductility. V

. Moreover. the resulting surface, when viewed with a ma nifying glassis minutely ebbled. which is a desirable characteristic. as a "pebbledsurface retains die lubricantswhich maybe-used during forming or drawingoperations and provides an excellent surface for the retention oradherence of paint or other coatings which may be applied to thefinished product.

It is therefore a further object of the present invention to provide amethod of pickling high manganese steel with an acid for the removal ofscale and brittle surface metal without etching or pitting the metal.

It is also an object of the present invention to provide a new treatmentof. high manganese steel utilizing a weak and relatively inexpensiveacid for scale and brittle surface metal removal.

Likewise, it is an object of the present invention to provide a methodof acid pickling high manganese steel, which not only removes scale andbrittle surface metal, but which also partially or totally removesseams, scratches, scale patterns and surface defects or imperfections,leaving a bright or lustrous, smooth, minutely pebbled surface.

Furthermore, it is an object of the present invention to provide a newtreatment of high manganese steel by which the surface appearance,toughness, strength, ductility and drawing qualities of the steel areimproved.

Also it is an object of the present invention to provide anew method ofmaking cold rolled, light gauge, high manganese strip or stripsheetsteel more quickly and more cheaply than it has been producedpreviously, by enabling the satisfactory commercial production thereofin accordance with strip practice.

And finally, it is an object of the present invention to provide a coldrolled, high manganese,

strip and sheet 'steel product having a bright, lustrous, relativelysmooth, pebbled surface, free of brittleness and having high ductility."

These and other objects may be obtained, the

stated results achieved, and the described diiiiculties overcome by themethods, steps, products, treatments, and discoveries whichcomprise thepresent invention, the nature of whichis set forth in the followinggeneral statement, and a preferred. embodiment of which is set forth inthe following description, and which are particularly anddistinctlypointed out and set .forth in the appended claims forming parthereof.

The nature of the present improvements may be stated in general terms aspreferably including, in the treatment of high manganese, high carbonsteel, which has been heat treated to soften the same and to restoreductility, the step of sub- .iecting the same to the action of a bath ofa dilute aqueous solution of nitric acid, say up to 5% nitric acid byweight in water, at a temperature of about 140" to 200 F. for asuflicient' time to remove scale on the surfaces thereof and to removebrittle surface metal layers thereon resulting from heat treatment, soas to provide a bright, ductile, product free of brittle surface layers,scratches, scale patterns, and roughness.

The manufacture of high manganese steel strips will be described. indetail as an embodi- -ment'of the way in which the method of the intosay .079 inch in thickness.

maximum allowable reduction, which is approxi- I mately at 25%reduction" for this type of steel,

The strip is then softened by heating it up to 1800 to 1900 F.,preferably about 1850 F., and is then quenched in water. This treatmentmay be termed austenitizing and dissolves brittle carbides, retainscarbon in solid solution, and renders the steel as fully austenitic asis possible.

The steel may then be pickled in any usual way for scale removal, or maybesubjected to the special pickling treatment of the present inventionto be later described in detail.

The steel may then be cold rolled to maximum allowable reduction, whichis approximately 25% reduction, down to say .058 inch in thickness,after which it is again heat treated and quenched. in the manner justdescribed, followed by a usual pickling operation to remove scale, or bythe should be replenished with small additions of nitric acid.

When the concentration of dissolved metal in may be made to the bath ifdesired.

special pickling operation of the present invention.

Thesteel may then be again cold rolled to maximum allowable reduction,say to .044 inch in thickness, which is one gauge of high manganesesteel desired for helmet stock, after which it is heat treated. asstated, and if no more cold rolling is to be done, the cold rolled heattreated strip steel must then, in accordance with the present invention,be subjected to the special pickling operation of the present invention.

The special pickling operation may be used for any one of the picklingoperations following any cold rolling step. The steel may be furthercold rolled to lighter gauges such as .033 inch thick stock. In anyevent, the last pickling operation following the last cold rolling stepmust be the special pickling operation-now to be described, in order toobtain the benefits of the discoveries and results of the presentinvention.

As previously stated, in addition to forming scale, the result of heattreatment utilized after each cold rolling operation. is to decarburizethe surface layers of the steel, which apparently results in theformation of ensilon and alpha iron or both in the surface layers uponQuenching.

These undesirable surface layers are removed after heat treatment by thespecial pickling operation of the invention which may be carried out bypassing the strip through and subjecting it to the action of a dilutesolution of up to about which the metal has been subjected, and t e ma--terial will have to remain in the nitric acid bath longer for removingthe undesirable surface layers.

If the acid concentration is too weak, the attack is too slow and alonger time .of treatment in the bath will be required. If the acidconcentration is too high, the attack is too vigorous and excessivefumes are formed. An increasein the temperature of the bath increasesthe rate of attack; and again, excessive fumes result when thetemperature is too high.

The pickling and metal removal act on of a fresh solution increases whenthe amount of dissolved iron is slightly increased; butthereafte whenthe iron content of the bath is considerably increased. the action issluggish and the bath The action of the bath can be speeded up, if

desired, by using the bath as an electrolyte, and

by using the metal as an anode and passing electric current through thesolution to electrolytically treat the metal for removing scale andbrittle surface metal layers. Likewise, the electrolytic treatment maybe carried out by using the strip as a bi-polar electrode, in accordancewith the method set forth in the L. H. Wilson Patent No. 2,197,653.

If the bath is operated as an electrolyte the temperature may bemaintained at about 60 F. to 150 preferably. about 120 F. for therequired length of time. With a comparatively small current density, aslightly used bath will dissolve the metal electrolytically at 120 F. atthe be loosened by passing the strip through a dilute solution ofsulphuric acid or hydrochloric acid in water, or through any other usualcleaning solution, prior to the scrubbing operation. After the scrubbingoperation, the strip is finally washed in water and dried by hot airblasts or other means, and oiled if desired. A

- When high manganese, high carbon steeltrips are made as outlinedabove, without using the special pickling operation of the presentinvention, the finished material is generally soft, well annealed, andfree from carbides: but the surfaces are poor, the ductility is lowerthan desired, and a granulated or "frosty condition occurs when thematerial is drawn.

Hardness tests made with various penetrations or depths to show therelative hardness at various depths, indicated that the brittle surfacelayer, is not necessarily due to a hard constituent such as martenite. v

Samples were reannealed to determine whether or not the ductility couldbe improved by changes in the heat treating practice; but these tests indicated that changes in the heattreatmentdid I not improve ductility orliminatethe brittle or frosty surface conditions. These and other'testsdid'showrlaow'ever, that excessive oxidation of the surface and' surfaceembrittlement go hand in hand; and analysis.

of the .rnetal at various depths indicated that the carbon content ofthe surface metal was matea Samples having decarburized surfaces wereagain carburized-to materially increase the carbon content of thesurface layers and were then heat treated so that the carbon contentofthe surface layers was approximately that of the core, somewherebetween 1% and 1.5% carbon. The result was exceptionally good ductilityand a disappearance of the frosty surface conditions, indicating thatthe brittle surface is produced by depletion of the surface metalcarbon. However, it is not feasible in the production of high manganese,high carbon steel strips to eliminate surface embrittlement by acarburizing step following the .final cold'rolling. and heat treatment.

Decarburization alone does not explain the exthe removal of the brittlesurface layers due to the precipitation of alpha or epsilon iron orboth,

istence of surface embrittlement found in heat treated Hadfieldf steel.The phenomenon is apparently due to some radical structural change,which occurs coincident with the lowering or depletion of the carboncontent. As previously stated, when the carbon is materially lowered,austenite is not completely retained upon.cooling, but some of itdecomposes into alpha iron or epsilon iron, or both, at approximately400 F. These alpha and epsilon phases, especially the latter, must beexpected to be brittle, and the brittle surface layer probably consistsof three phasesalpha, gamma and epsilon irons; which is apparently thestructural change occurring with a depletion of the carbon in thesurface found that the surface metal and core metal responds differentlyto different etchants, and careful etching technique reveals that thedecar burized surface layer contains considerable Wid- --manstattestructure.

As to the'heat treating cycle, the [best results will be obtained andthe surface embrittlement maintained at a by avoiding oxidizingatmospheres, by annealing at the lowest temperature and fastest speedrequired to dissolve the carbides, and by quenching immediately inwater. Oxidizing atmospheres and violently agitated atmosphere tend toincrease scaling and surface embrittlement, and the best atmosphere is aquiet atmosphere of reducing gas.

Hadfield" steels must be heated to above the upper critical point, 1800"F. to, 1900 F. in order to dissolve carbides and produce austenite, buthigher temperatures than absolutely necessary increase caling-andsurface embrittlement. In general two minutes heat treatment at about1850 F., or between 1850 F. and 1900 F., one minute, coming totemperature and a one min! ute soak, is suflicient. Increasing lifetimein the furnace vproducesheavier scale and increases surfaceembrittlement and frost.

in the decarburized surface metal, the brittle layers being extremelydetrimental to strength and ductility; and because of the formation of asmooth lustrous surface of minutely pebbled (contour.

' The final product is fully austenitic in character with the relatedcharacteristics of austenitic manganese steel fully developed entirelythroughout the metal of the strip and the improved surface is a superiorbase for die lubricants in forming operations, and for paints or othercoatings applied to the finished product. This surface is much moresmooth and ductile than the usual pickled surfaces which contain manysharp projections and irregularities detrimental to strength andductility. Actual tests have shown that the treatment increases theductility of the material as much as 10% or more.

We therefore term the special pickling treatment of the presentinvention a pickle polishing strip steel or strip sheets in accordancewith strip practice, the present invention is also applicable tothemanufacture of other high manganese steel products such as sheetsteel inaccordance with sheet practice, wherein the pickle polishing operationwould be carried out following the final cold rolling and heat treatmentoperations,

of which there may be only one ortwo in sheet practice, to remove thebrittle surface layers .and provide a finished product having the samecharacteristics as those discussed herein in connection with themanufacture of strips.

Quenching in water is better than air cooling.

The result of the special pickling treatment of I the present inventionfollowingvheat treatment, is to increase the toughness, strength andduetility of the metal because of the removal of scale,

seams, scratches and other surface imperfections detrimental to strengthandductility; because-of 75 Accordingly, when the term strips is usedherein and in .the'appended claims, the term is intended to include thetreatment of hot or cold rolled high manganese steel strips, stripsheets,

sheets, plates, bars, rounds, wire, forgings orcastings and the like, inorder to remove scale, seams, scratches, brittle surface layers andother surface imperfections, to provide a surface much improved overthat produced by usual pickling methods.

When the term "strips of the lighter gauges is used herein and in theappended claims, that term is intended to include strips, stripsheets,sheets and the like, asthin as .044 to .033 inch in thickness, orthinner, and which require forpro "Hadfield type, which generallycontains from 10% to 20% manganese and from 1% to 1.5%

carbon, with possible additions of small percent- 1 ages of otheralloying elements, such as nickelare'sometimes added to:

.andchromium, which Hadfield" steels.

Thus, the present inmprovements and discoveries provide for the readyand economical manufacture of high manganese steel strips of the lightergauges having extremely high ductility characteristic of fullyaustenitic manganese steel, and having a bright color and sheen adaptedfor forming and bending operations because of the minutely "pebbledtexture thereof, the pebbled projections being smooth and rounded so asto appear bright to the naked eye.

Having now described the .features of the invention, and an embodimentof steps by which the invention may be carried out, and the advantagesand results attained by the invention, and the new discoveries modemconnection with the treatment of high manganese steel; the new anduseful methods, steps, treatments, arrangements and products, are setforth in the appended claims.

I claim;

1. In the manufacture of thin gauge, cold rolled, heat treated, highmanganese, high carbon,

strip steel and the like; the step of subjecting such steel strip aftercold rolling and austenitizing heat treatment to the action of a diluteaqueous solution of up to approximately 5% by weight nitric acid toremove scale and surface layers of brittle metal formed by heattreatment, and to render the steel'fully austenitic throughout.

- carbon, strip steel by heating the same to between 2. In themanufacture of heat treated, high manganese, high carbon, strip steeland the like; the steps of heat treating high manganese, high carbon,strip steel by heating the same to between 1800 F. and 1900 F., and thenquenching in water; and then pickle polishing the steel in a diluteaqueous solution of up to approximately 5% by weight nitric acid toremove scale and surface metaland to form smooth bright surfacesthereon.

3. The method of treating thin gauge, cold rolled, austenitized, highmanganese, high carbon, strip steel and the like, to remove scale, I

brittle surface layers, scratches, scale patterns and surface defects,and to increase the ductility thereof; which includes pickling suchsteel in a dilute aqueous solution of up to approximately 5% by weightnitric acid to remove scale, and continuing the pickling operation touniformly dissolve the thin layers of brittle surface metal thereon, andto form smooth, bright, lustrous, minutely pebbled surfaces thereon.

4. In the manufacture of thin gauge, cold rolled, heat treated, highmanganese, high carbon, strip steel and the like; the steps of heattreating cold rolled, high manganese, high earbon, strip steel byheating the same for up to about 2 minutes to between 1800 F. and 1900F., and then quenching inwater; and then removing surface metal layersof the strip by pickling in a dilute aqueous solution of up toapproximately 5% by weight nitric acid to eliminate decarburized surfacemetal containing alpha and epsilon iron formed by the heat treatment.

5. In the manufacture of heat treated, high manganese, high carbon,strip steel and the like; the steps of heat treating high manganese,high carbon, strip steel by heating the same to between 1800 F. and 1900F., and then quenching in water; and then pickle polishing the steel ina solution of 1% to 5% by weight of nitric acid in water at atemperature of from F. to 200 F. for 1 to 6 minutes to remove'scale andsurface metal.

6. In the manufacture of heat treated, high manganese, high carbon,strip steel and the like; the steps of heat treating high manganese;high 1800 F. and 1900 E, and then quenching in water; and then picklepolishing the steel in 'a solution of approximately 5% nitric acid byweight in water at a temperature of about 180 F. for from 1 to 6 minutesto remove scale and surface metal and to form smooth bright surfacesthereon.

'7. In the manufacture of heat treated, high manganese, high carbon,strip steel and the like; the steps of heat treating high manganese,high carbon, strip steel by heating the same for upwards of 2 minutes ata temperature of from 1850 F. to 1900" F., and then quenching in water;and then pickle polishing the steel in a dilute aqueous solution of upto 5% nitric acid by weight in water toremove scale and surface metaland to form smooth, bright surfaces thereon.

8. In the manufacture of heat treated, high manganese, high carbon,strip steel and the like; the steps of heat treating high manganese,high carbon, stripsteel by heating the same for about 2 minutes at atemperature of about 1850 F.,

" and then quenching in water; and then pickle polishin thev steel ina-solution of 1% to 5% nitric acid by weight in water at a temperatureof aboutv 180 F. for from 1 to 6 minutes to remove scale and surfacemetal and to form smooth bright surfaces thereon.

CARL W. WEESNER.

W. BRUCE LEFF'INGWELL.

